Microelectronic devices are fabricated on semiconductor (e.g. silicon) wafers using a variety of techniques, e.g. including deposition techniques (CVD, PECVD, PVD, etc.) and removal techniques (e.g. chemical etching, CMP, etc.). Semiconductor wafers may be further treated in ways that alter their mass, e.g. by cleaning, ion implantation, lithography and the like.
Depending on the device being manufactured, each semiconductor wafer may be passed sequentially through hundreds of different processing steps to build up and/or to remove the layers and materials necessary for its ultimate operation. In effect, each semiconductor wafer is passed down a production line. The nature of semiconductor manufacturing means that certain processing steps or sequences of steps in the production flow may be repeated in a similar or identical fashion. For example, this may be to build up similar layers of metal conductors to interconnect different parts of the active circuitry.
To ensure consistency and interoperability of semiconductor equipment used in different factories, standards are adopted throughout the majority of the semiconductor manufacturing industry. For example, standards developed by Semiconductor Equipment and Materials International (SEMI) have a high degree of market uptake. One example of standardisation is the size and shape of the semiconductor (silicon) wafers: typically for volume production they are discs having a diameter of 300 mm. However, some semiconductor (silicon) wafers (typically used in older factories) are discs having a diameter of 200 mm.
The cost and complexity of the processing steps required to produce a completed silicon wafer, together with the time that it takes to reach the end of the production line where its operation can be properly assessed, has led to a desire to monitor the operation of the equipment on the production line and the quality of the wafers being processed throughout processing, so that confidence in the performance and yield of the final wafers may be assured.
Wafer treatment techniques typically cause a change in mass of the semiconductor wafer (e.g. at or on the surface of the semiconductor wafer or in the bulk of the semiconductor wafer). The configuration of the changes to the semiconductor wafer are often vital to the functioning of the device, so it is desirable for quality control purposes to assess wafers during production in order to determine whether they have the correct configuration.
Specialist metrology tools may be used within the production flow so that monitoring is conducted soon after the relevant process of interest and usually before any subsequent processing, i.e. between processing steps.
Measuring the change in mass of a wafer either side of a processing step is an attractive method for implementing product wafer metrology. It is relatively low cost, high speed and can accommodate different wafer circuitry patterns automatically. In addition, it can often provide results of higher accuracy than alternative techniques. For example, on many typical materials, thicknesses of material layers can be resolved down to an atomic scale. The wafer in question is weighed before and after the processing step of interest. The change in mass is correlated to the performance of the production equipment and/or the desired properties of the wafer.
Processing steps carried out on semiconductor wafers can cause very small changes in the mass of the semiconductor wafer, which it may be desirable to measure with high accuracy. For example, removing a small amount of material from the surface of the semiconductor wafer may reduce the mass of the semiconductor wafer by a few milligrams, and it may be desirable to measure this change with a resolution of the order of ±100 μg or better. Semiconductor wafer metrology methods and apparatus that are capable of measuring the change in mass of a semiconductor wafer to a resolution of around ±10 μg are in development, and methods and apparatus with a resolution of around ±100 μg are commercially available.
At these high levels of measurement accuracy, errors in the measurement output caused by temperature variations in the semiconductor wafers being measured or in the temperature of the measurement balance may become significant. For example, a temperature difference of approximately 0.005° C. between the semiconductor wafer and (a part of) the measurement balance or enclosure may cause an error of approximately 5 μg in the determined mass (or change in mass) of the semiconductor wafer. Variations in temperature between different parts of the measurement apparatus (i.e. temperature non-uniformity), e.g. caused by a heat load from semiconductor wafers being measured using the measurement apparatus, will cause errors in the measurement output. In addition, if the semiconductor wafers have a higher temperature than a measurement enclosure of the measurement apparatus, air currents (e.g. convection currents) may be generated in the air in the measurement enclosure, which may affect the measurement output. In addition, the air in the measurement enclosure may be heated, changing its density and pressure and therefore the buoyancy force exerted on the semiconductor wafer by the air. This may also affect the measurement output. The magnitudes of these effects are generally considered insignificant and are ignored (or not detected) in lower accuracy mass measurements, for example measurements performed with a resolution of the order of milligrams.
Temperature changes occurring slowly over a relatively long period of time (e.g. of the order of hours) may be essentially accounted for by periodically calibrating the measurement apparatus, or may be essentially subtracted out by performing comparative measurements. However, temperature changes occurring more rapidly (e.g. due to a high heat load from a plurality of semiconductor wafers) may be more difficult to account for or to subtract out in the same way.
The temperature of a semiconductor wafer immediately after it has been processed in a production line may be 400-500° C. or higher. After processing the semiconductor wafer may be loaded into a Front Opening Unified Pod (FOUP) together with other recently processed semiconductor wafers (e.g. 25 in total) for transportation between different processing locations of the production line. When the FOUP arrives at a different processing location, for example a weighing device for weighing the semiconductor wafers, the temperature of the semiconductor wafers may still be high, for example 70° C. or higher. In contrast, the temperature of the processing location, e.g. the weighing device, may be approximately 20° C. Therefore, there may be a significant temperature difference between the semiconductor wafers and the weighing device. As discussed above, a significant temperature difference between the semiconductor wafer and the weighing device may cause convection currents in the weighing device, and/or changes in the buoyancy force experienced by the semiconductor wafer (due to changes in the air density and/or pressure) and/or thermal variation (i.e. temperature changes and/or temperature non-uniformity) in a weighing balance of the weighing device, which may cause errors in the weight measurements. For high accuracy weight measurements, even errors caused by very small temperature differences (e.g. less than 1° C., for example 0.001° C.) may be significant (e.g. detectable).
WO 02/02449 describes a semiconductor wafer metrology method that aims to reduce errors in the measurement output caused by temperature variations in the measurement balance or of the semiconductor wafers being measured. In the method described in WO 02/02449 a semiconductor wafer is removed from a Front Opening Unified Pod (FOUP) and placed on a passive thermal transfer plate that is thermally coupled to a chamber of the weighing apparatus before it is placed on a measurement area of the weighing apparatus. The passive thermal transfer plate equalises the temperature of the semiconductor wafer to the temperature of the chamber to within ±0.1° C. This temperature equalisation may reduce the possibility of any convection currents arising within the balance, which would cause errors in the measurement output, and also may reduce any thermal variation in the balance itself, which would also cause errors in the measurement output. This method therefore may make the measurement output more accurate, relative to a method in which there is no temperature equalisation of the semiconductor wafer before taking the measurement.